Self-directed growth of molecular nanostructures on silicon

Surface-state engineering for interconnects on H-passivated Si(100)

Surface-state engineering strategies for atomic-size interconnects on H-passivated Si(100) surfaces are explored. The well-known simple interconnect formed by removing H-atoms from one of the Si atoms per dimer of a dimer row along the Si(100) surface is poorly conducting. This is because one-dimensional-like instabilities open electronic gaps. Here, we explore two strategies to reduce the instabilities: spacing the dangling bonds with H atoms and changing the geometry by increasing the lateral size of the wires. The resulting wires are evaluated using density functional theory. Surprisingly, zigzag dangling-bond wires attain atomically confined conduction properties comparable with the conduction of free-standing metallic monatomic wires. These results hint at band-engineering strategies for the development of electronically driven nanocircuits.

Photoresponsive molecules in well-defined nanoscale environments

Stimuli-responsive molecules are key building blocks of functional molecular materials and devices. These molecules can operate in a range of environments. A molecule's local environment will dictate its conformation, reactivity, and function; by controlling the local environment we can ultimately develop interfaces of individual molecules with the macroscopic environment. By isolating molecules in well-defined environments, we are able to obtain both accurate measurements and precise control. We exploit defect sites in self-assembled monolayers (SAMs) to direct the functional molecules into precise locations, providing a basis for the measurements and engineering of functional molecular systems. The structure and functional moieties of the SAM can be tuned to control not only the intermolecular interactions but also molecule-substrate interactions, resulting in extraction or control of desired molecular functions. Herein, we report our progress toward the assembly and measurements of photoresponsive molecules and their precise assemblies in SAM matrices.

Gas-phase hydrosilylation of plasma-synthesized silicon nanocrystals with short- and long-chain alkynes

Surface passivation of Si nanocrystals (NCs) is necessary to enable their utilization in novel photovoltaic and optoelectronic devices. Herein, we report the surface passivation of plasma-synthesized, H-terminated Si NCs via gas-phase hydrosilylation using a combination of short- and long-chain alkynes. Specifically, using in situ attenuated total reflection Fourier transform infrared spectroscopy, we show that a sequential exposure of the Si NC surface to acetylene and phenylacetylene results in a surface alkenyl coverage of ∼58%, which is close to the theoretical maximum of ∼55% and ∼60% predicted for alkyl- and alkenyl-terminated Si(111) surfaces, respectively. We attribute this unprecedented high surface hydrocarbon coverage to the combination of short- and long-chain alkynes that reduce the steric hindrance on the surface, higher reactivity of 1-alkynes versus 1-alkenes of the same chain length, and the smaller van der Waals radius of the alkenyl groups compared to the alkyl groups. Unlike 1-alkenes, 1-alkynes also react with the surface to form the 1,1- and 1,2-bridge structures via the bis-hydrosilylation reaction. However, our data clearly show that this reaction pathway cannot account for the enhanced surface coverage in the sequential exposure experiments, since exposure of the surface to just acetylene or phenylacetylene results in an almost identical surface coverage due to the 1,1- and 1,2-bridge sites.

Negative differential resistance in a hybrid silicon-molecular system: resonance between the intrinsic surface-states and the molecular orbital

It has been a long-term desire to fabricate hybrid silicon-molecular devices by taking advantages of organic molecules and the existing silicon-based technology. However, one of the challenging tasks is to design applicable functions on the basis of the intrinsic properties of the molecules, as well as the silicon substrates. Here we demonstrate a silicon-molecular system that produces negative differential resistance (NDR) by making use of the well-defined intrinsic surface-states of the Si (111)-√3 × √3-Ag (R3-Ag/Si) surface and the molecular orbital of cobalt(II)-phthalocyanine (CoPc) molecules. From our experimental results obtained using scanning tunneling microscopy/spectroscopy, we find that NDR robustly appears at the Co(2+) ion centers of the CoPc molecules, independent of the adsorption configuration of the CoPc molecules and irrespective of doping type and doping concentration of the silicon substrates. Joint with first principle calculations, we conclude that NDR is originated from the resonance between the intrinsic surface-state band S(1) of the R3-Ag/Si surface and the localized unoccupied Co(2+)d(z(2)) orbital of the adsorbed CoPc molecules. We expect that such a mechanism can be generally used in other silicon-molecular systems.

Noncovalent bicomponent self-assemblies on a silicon surface

Two-dimensional supramolecular multicomponent networks on surfaces are of major interest for the building of highly ordered functional materials with nanometer-sized features especially designed for applications in nanoelectronics, energy storage, sensors, etc. If such molecular edifices have been previously built on noble metals or HOPG surfaces, we have successfully realized a 2D open supramolecular framework on a silicon adatom-based surface under ultrahigh vacuum with thermal stability up to 400 K by combining molecule-molecule and molecule-silicon substrate interactions. One of these robust open networks was further used to control both the growth and the periodicity of the first bicomponent arrays without forming any covalent bond with a silicon surface. Our strategy allows the formation of a well-controlled long-range periodic array of single fullerenes by site-specificity inclusion into a bicomponent supramolecular network.

A single-atom transistor

The ability to control matter at the atomic scale and build devices with atomic precision is central to nanotechnology. The scanning tunnelling microscope can manipulate individual atoms and molecules on surfaces, but the manipulation of silicon to make atomic-scale logic circuits has been hampered by the covalent nature of its bonds. Resist-based strategies have allowed the formation of atomic-scale structures on silicon surfaces, but the fabrication of working devices-such as transistors with extremely short gate lengths, spin-based quantum computers and solitary dopant optoelectronic devices-requires the ability to position individual atoms in a silicon crystal with atomic precision. Here, we use a combination of scanning tunnelling microscopy and hydrogen-resist lithography to demonstrate a single-atom transistor in which an individual phosphorus dopant atom has been deterministically placed within an epitaxial silicon device architecture with a spatial accuracy of one lattice site. The transistor operates at liquid helium temperatures, and millikelvin electron transport measurements confirm the presence of discrete quantum levels in the energy spectrum of the phosphorus atom. We find a charging energy that is close to the bulk value, previously only observed by optical spectroscopy.

Functionalization of a self-assembled monolayer driven by low-energy electron exposure

Self-assembled monolayers (SAMs) of 10-undecene-1-thiol on Au were functionalized with nitrogen-containing groups using an approach in which multilayer ammonia (NH(3)) films were deposited at low temperature onto the SAMs and subsequently exposed to 15 eV electrons. The result of this process was investigated after removal of the remaining NH(3) by annealing to room temperature using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). HREELS shows that the CC double bonds disappear during electron exposure, while XPS gives evidence that about 25% of the terminal double bonds of the SAM were functionalized. Also, XPS shows that a sufficiently thick NH(3) layer protects the underlying SAM from electron-induced damage. The process suggested here thus represents a particularly gentle approach to the functionalization of ultrathin molecular layers. Thermal desorption spectrometry (TDS) and electron-stimulated desorption (ESD) experiments on condensed layers of NH(3) reveal production of N(2) but show that significant amounts of the initial NH(3) as well as N(2) produced during electron exposure desorb. Hydrogen released upon formation of N(2) is held responsible for the reduction of double bonds and protection of the SAMs from damage.

NITRILES ADSORBED ON Si(001) AT 300 K STUDIED VIA SYNCHROTRON RADIATION CORE-ELECTRON SPECTROSCOPIES

This paper focuses on the use and value of XPS and NEXAFS spectroscopies to unveil the nature of the chemical bond of various bifunctional nitrile molecules adsorbed on Si (001) 2×1 at 300 K. The adsorption modes are also discussed in the light of recent theoretical publications devoted to optimized geometries and reaction paths of these molecules on Si (001).

In situ gas-phase hydrosilylation of plasma-synthesized silicon nanocrystals

Surface passivation of semiconductor nanocrystals (NCs) is critical in enabling their utilization in novel optoelectronic devices, solar cells, and biological and chemical sensors. Compared to the extensively used liquid-phase NC synthesis and passivation techniques, gas-phase routes provide the unique opportunity for in situ passivation of semiconductor NCs. Herein, we present a method for in situ gas-phase organic functionalization of plasma-synthesized, H-terminated silicon (Si) NCs. Using real-time in situ attenuated total reflection Fourier transform IR spectroscopy, we have studied the surface reactions during hydrosilylation of Si NCs at 160 °C. First, we show that, during gas-phase hydrosilylation of Si NCs using styrene (1-alkene) and acetylene (alkyne), the reaction pathways of the alkenes and alkynes chemisorbing onto surface SiH(x) (x = 1-3) species are different. Second, utilizing this difference in reactivity, we demonstrate a novel pathway to enhance the surface ligand passivation of Si NCs via in situ gas-phase hydrosilylation using the combination of a short-chain alkyne (acetylene) and a long-chain 1-alkene (styrene). The quality of surface passivation is further validated through IR and photoluminescence measurements of Si NCs exposed to air.

Charge control of surface dangling bonds using nanoscale Schottky contacts

Titanium silicide (TiSi2) nanoscale Schottky contacts were prepared on hydrogen-terminated n-type Si (100) surfaces. The Schottky contact created a region of upward band bending surrounding the TiSi2 contacts. The surface band bending was observed as a sloping surface depression using the scanning tunneling microscope. Scanning tunneling spectroscopy measurements also show shifted I/V data consistent with upward band bending. Charge control of dangling bonds was accomplished through a distance relationship between the dangling bond and the TiSi2 contact. The lowered chemical potential in the near contact region removes the ability of dangling bonds to become negatively charged while dangling bonds outside the close contact region remain fully charged. Methods to actively control dangling bond charge states are discussed.

Indications of field-directing and self-templating effects on the formation of organic lines on silicon

It has previously been shown that multimolecular organic nanostructures form on H-Si(100)-2×1 via a radical mediated growth process. In this mechanism, growth begins through the addition of a molecule to a silicon surface dangling bond, followed by the abstraction of a neighboring H atom and generation of a new dangling bond on the neighboring site. Nanostructures formed by this mechanism grow along one edge of a dimer row. Here, we explored the possibility of using lithographically prepared, biased metal contacts on the silicon surface to generate an electric field that orients molecules during the growth process to achieve growth in the perpendicular-to-row direction. The formation of some nanostructures in a direction that was nearly perpendicular to the dimer rows was achieved, whereas such features were not formed in the absence of the field. Analysis of the scanning tunneling microscopy images suggests that the formation of these nanostructures may involve self-templating effects in addition to dangling bond diffusion rather than a straightforward addition∕abstraction mechanism. These initial results offer some indication that a molecular pattern writer can be achieved.

Electron Transport Suppression from Tip-π State Interaction on Si(100)-2 × 1 Surfaces

We investigate the electron transport between a scanning tunneling microscope tip and Si(100)-2 × 1 surfaces with four distinct configurations by performing calculations using density functional theory and the nonequilibrium Green's function method. Interestingly, we find that the conducting mechanism is altered when the tip-surface distance varies from large to small. At a distance larger than the critical value of 4.06 Å, the conductance is increased with a reduction in distance owing to the π state arising from the silicon dimers immediately under the tip; this in turn plays a key role in facilitating a large transmission probability. In contrast, when the tip is closer to the substrate, the conductance is substantially decreased because the π state is suppressed by the interaction with the tip, and its contribution in the tunneling channels is considerably reduced.

Small-sized silicon nanoparticles: new nanolights and nanocatalysts

Owing to their abundant unique properties and ready compatibility with Si microelectronic technology, Si nanostructures are becoming one of the most important classes of nano semiconductors. Particularly, small-sized Si nanoparticles possess distinctive photoluminescence (PL), biocompatibility, and active surface properties. In this review, we focus on the synthesis and PL properties of small-sized Si nanoparticles and their potential applications in the fields of bioimaging and nanocatalysis. We further highlight major challenges and promises in this area.

Large-scale patterning of zwitterionic molecules on a Si(111)-7 × 7 surface

The formation of a large scale pattern on Si(111)-7 × 7 reconstruction is still a challenge. We report herein a new solution to achieve this type of nanostructuration by using of zwitterionic molecules. The formation of a large-scale pattern is successfully obtained due to the perfect match between the molecular geometry and the surface topology and to electrostatic interactions between molecules and surface. The adsorption is described by high-resolution scanning tunneling microscopy (STM) images and supported by density functional theory and STM calculations.

Structure and stability of one-dimensional o-phthalaldehyde lines on the Si(100)-2 × 1:H surface

We present the structural models for the o-phthalaldehyde (OP) molecular lines on the H-terminated Si(100) surface which were recently observed by scanning tunnelling microscopy. Our first-principles density-functional theory calculations show that the formation of OP lines is not only kinetically more facile but also thermodynamically more stable than those of previously reported alkene lines.

Enantioselective adsorption of surfactants monitored by ATR-FTIR

The selectivity of adsorption of chiral surfactants to a chiral monolayer at the solid-liquid interface was studied using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). One enantiomer of the chiral surfactant was deuterated, which causes a change in the IR absorption frequency, and allows independent measurement of the adsorption of each molecule. Both the surfactant, N-lauroyl phenylalanine (NLP), and the chiral monolayer, N-L-phenylalaninoyl, 11-undecyl-silicon, were amino acid derivatives. An enantiomeric excess of 56 +/- 22% of the L over D was observed for adsorption to the interface between a carbon tetrachloride solution containing a quasi-racemate of N-lauroyl phenylalanine and the N-L-phenylalaninoyl, 11-undecyl monolayer film on silicon. In contrast, equimolar adsorption occurred from an equimolar mixture of hydrogenated and deuterated forms of the L surfactant. The measured enantiomeric excess strongly depended on the density of chiral surface groups: the higher the density of chiral groups on the surface, the better the enantiodiscrimination, even though the total adsorption was roughly constant. This nonlinear behavior indicates that more than one chiral surface group is required for significant selectivity.

Metallic nanostructure formation limited by the surface hydrogen on silicon

Constant miniaturization of electronic devices and interfaces needed to make them functional requires an understanding of the initial stages of metal growth at the molecular level. The use of metal-organic precursors for metal deposition allows for some control of the deposition process, but the ligands of these precursor molecules often pose substantial contamination problems. One of the ways to alleviate the contamination problem with common copper deposition precursors, such as copper(I) (hexafluoroacetylacetonato) vinyltrimethylsilane, Cu(hfac)VTMS, is a gas-phase reduction with molecular hydrogen. Here we present an alternative method to copper film and nanostructure growth using the well-defined silicon surface. Nearly ideal hydrogen termination of silicon single-crystalline substrates achievable by modern surface modification methods provides a limited supply of a reducing agent at the surface during the initial stages of metal deposition. Spectroscopic evidence shows that the Cu(hfac) fragment is present upon room-temperature adsorption and reacts with H-terminated Si(100) and Si(111) surfaces to deposit metallic copper. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) are used to follow the initial stages of copper nucleation and the formation of copper nanoparticles, and X-ray energy dispersive spectroscopy (XEDS) confirms the presence of hfac fragments on the surfaces of nanoparticles. As the surface hydrogen is consumed, copper nanoparticles are formed; however, this growth stops as the accessible hydrogen is reacted away at room temperature. This reaction sets a reference for using other solid substrates that can act as reducing agents in nanoparticle growth and metal deposition.

Self-assembling siloxane bilayer directly on SiO2 surface of micro-cantilevers for long-term highly repeatable sensing to trace explosives

This paper presents a novel sensing layer modification technique for static micro-cantilever sensors that detect trace explosives by measuring specific adsorption-induced surface stress. For the first time, a method of directly modifying a siloxane sensing bilayer on an SiO(2) surface is proposed to replace the conventional self-assembled monolayers (SAMs) of thiols on Au to avoid the trouble from long-term unstable Au-S bonds. For modifying the long-term reliable sensing bilayer on the piezoresistor-integrated micro-cantilevers, a siloxane-head bottom layer is self-assembled directly on the SiO(2) cantilever surface, which is followed by grafting another explosive-sensing-group functionalized molecule layer on top of the siloxane layer. The siloxane-modified sensor has experimentally exhibited a highly resoluble response to 0.1 ppb TNT vapor. More importantly, the repeated detection results after 140 days show no obvious attenuation in sensing signal. Also observed experimentally, the specific adsorption of the siloxane sensing bilayer to TNT molecules causes a tensile surface stress on the cantilever. Herein the measured tensile surface stress is in contrast to the compressive surface stress normally measured from conventional cantilever sensors where the sensitive thiol-SAMs are modified on an Au surface. The reason for this newly observed phenomenon is discussed and preliminarily analyzed.

Beyond Cassie's law: a theoretical and experimental study of mixed alkyl monolayers

Within this study, the influence of ester groups in mixed monolayers on the surface properties will be discussed. Detailed investigations on the macroscopic and microscopic characteristics on mixed monolayers with different content of ester groups in an alkyl surrounding are done by contact angle measurements and atomic force spectroscopy. Density functional theory (DFT) calculations show a statistical distribution and a directed orientation of the ester molecules. In the experiments an increasing amount of ester groups leads to a fast increasing polarity followed by a nearly constant polarity in the regime of 25% and 40% of ester in the monolayer and a further increase at higher amounts of ester groups, which clearly differ from the behavior expected by Cassie. By DFT calculations it can be shown that water molecules form ring-like structures around the ester group. These solvent shells increase the hydrophilic fraction on the surface explaining the disproportional growth in the polarity of the monolayer. This rise in polarity is maximal for single ester groups (monomers) or dimers of esters. The amount of these monomers and dimers is estimated by Monte Carlo simulation showing clearly that the linear regime at fractions between 0.25 and 0.4 are caused by the transition from mainly monomers to mainly dimers. Thus, we show for the first time that adsorbed water molecules forming a solvent shell around hydrophilic groups in hydrophobic surroundings influence the surface properties of mixed monolayers on a macroscopic and microscopic scale which therefore must be taken into account when preparing, investigating, using and understanding such monolayers.

Characteristic IR C=C stretch enhancement in monolayers by nonconjugated, noncumulated unsaturated bonds

Control over and understanding of single-molecule covalent coatings becomes increasingly important in tailoring surfaces during the fabrication of nanoscale electrical or optical elements, such as organic field-effect transistors and light-emitting devices as well as microelectromechanical systems as the relevant feature sizes decrease. In this work, we develop a model based on IR spectra from public databases and DFT calculations that can be used to semiquantitatively assess the level of double bonds in monolayer coatings. We use the model to show the enhancement of the C=C vibrational mode due to silicon substitution and also from additional unsaturated bonds. Simple models for other functional groups in organic monolayers could be produced similarly.

Modelling components of future molecular devices

We discuss challenges involved in modelling different components of molecular devices and give several examples that demonstrate how computer modelling evolved over the last few years to become a comprehensive tool for designing molecules, predicting their adsorption and diffusion at surfaces, simulating atomic force microscopy imaging and manipulation of atoms and molecules at insulating surfaces and studying electron conduction in prototype molecular devices. We describe some of the computational techniques used for modelling adsorption, diffusion, imaging and manipulation of organic molecules at surfaces and challenges pertaining to these studies, give several examples of applications and discuss further prospects for theoretical modelling of complex organic molecules at surfaces.

Unanticipated C=C bonds in covalent monolayers on silicon revealed by NEXAFS

Interfaces are crucial to material properties. In the case of covalent organic monolayers on silicon, molecular structure at the interface controls the self-assembly of the monolayers, which in turn influences the optical properties and electrical transport. These properties intrinsically affect their application in biology, tribology, optics, and electronics. We use near-edge X-ray absorption fine structure spectroscopy to show that the most basic covalent monolayers formed from 1-alkenes on silicon retain a double bond in one-fifth to two-fifths of the resultant molecules. Unsaturation in the predominantly saturated monolayers will perturb the regular order and affect the dependent properties. The presence of unsaturation in monolayers produced by two different methods also prompts the re-evaluation of other radical-based mechanisms for forming covalent monolayers on silicon.